The present invention relates generally to variable voltage protection devices used to protect electronic circuits from overvoltage transients caused by lightning, electromagnetic pulses, electrostatic discharges, ground loop induced transients, or inductive power surges. The present invention relates particularly to materials of construction for variable voltage protection components and methods of making variable voltage protection components and devices.
Voltage transients can induce very high currents and voltages that can penetrate electrical devices and damage them, either causing hardware damage, such as semiconductor burnout, or electronic upset, such as transmission loss or loss of stored data. The voltage transients produce large voltage spikes with high peak currents (i.e, overvoltage). The three basic overvoltage threats are electrostatic discharge, line transients, and lightning. Electrostatic discharge typically occurs when static charge dissipates off the body of a person in direct physical contact with an operating electronic system or an individual component, such as an integrated circuit chip. Line transients are surges in AC power lines. Line transients can also occur due to closing a switch or starting a motor. Lightning strikes can strike stationary objects, such as a building, or mobile objects such as aircraft or missiles. Such strikes can suddenly overload a system""s electronics. At peak power, each of these threats is capable of destroying the sensitive structure of an integrated circuit chip.
Various overvoltage protection materials have been used previously. These materials are also known as nonlinear resistance materials and are herein referred to as voltage variable materials. In operation, the voltage variable material initially has high electrical resistance. When the circuit experiences an overvoltage spike, the voltage variable material quickly changes to a low electrical resistance state in order to short the overvoltage to a ground. After the overvoltage has passed, the material immediately reverts back to a high electrical resistance state. The key operational parameters of the voltage variable material are the response time, the clamp voltage, the voltage peak and peak power. The time it takes for the voltage variable material to switch from insulating to conducting is the response time. The voltage at which the voltage variable material limits the voltage surge is called the clamp voltage. In other words, after the material switches to conducting, the material ensures that the integrated circuit chip, for example, will not be subjected to a voltage greater than the clamp voltage. The voltage at which the voltage variable material will switch (under surge conditions) from insulating to conducting is the switch voltage. These materials typically comprise finely divided conductive or semiconductive particles dispersed in an organic resin or other insulating medium. For example, U.S. Pat. No. 3,685,026 (Wakabayashi, et al.), U.S. Pat. No. 4,977,357 (Shrier) and U.S. Pat. No. 4,726,991 (Hyatt et al.) disclose such materials.
Voltage variable materials and components containing voltage variable materials have been incorporated into overvoltage protection devices in a number of ways. For example, U.S. Pat. No. 5,142,263 and 5,189,387 (both issued to Childers et al.) disclose a surface mount device which includes a pair of conductive sheets and voltage variable material disposed between the pair of conductive sheets. U.S. Pat. No. 4,928,199 (Diaz et al.) discloses an integrated circuit chip package which comprises a lead frame, an integrated circuit chip protected by an electrode cover which is connected to ground on one side, and a variable voltage switching device including the voltage variable material connected to the electrode cover on the other side. U.S. Pat. No. 5,246,388 (Collins et al.) is directed to a device having a first set of electrical contacts that interconnect with signal contacts of an electrical connector, a second set of contacts that connect to a ground, and a rigid plastic housing holding the first and second set of contacts so that there is a precise spacing gap to be filled with the overvoltage material. U.S. Pat. No. 5,248,517 (Shrier et al.) discloses painting or printing the voltage variable material onto a substrate so that conformal coating with voltage variable material of large areas and intricate surfaces can be achieved. By directly printing the voltage variable material onto a substrate, the voltage variable material functions as a discreet device or as part of associated circuitry.
The above U.S. Patents referred to are incorporated herein by reference.
Although the prior art discloses various materials and devices, there is a continuing and long felt need to provide improved cost-effective voltage variable materials and devices of more consistent performance properties to prevent variations in the clamp voltage under various conditions in which the materials and devices are used.
This invention comprises in one aspect a variable voltage protection device which comprises a single layer of neat dielectric polymer or glass positioned between a ground plane and an electrical conductor of an electronic device. It has surprisingly been found that overvoltage protection can be effectively provided by such a polymer or glass layer, provided that the polymer or glass layer is sufficiently thin to provide the switching and the voltage clamping characteristics desired for a given protective device for a given electronic device. It has been found that for certain polymers the thickness must be less than about 1.6 mils and for other polymers the thickness must be less than about 0.8 mil, preferably less than about 0.5 mil and more preferably less than about 0.2 mil. For certain glasses the thickness must be less than about 1.6 mils, with thicknesses less than 0.8 mil preferred in many applications.
In another aspect of the present invention, it has been found that superior performance can be provided by a variable voltage protection component which comprises the combination of (a) a layer of variable voltage protection material comprising a binder containing conductive particles and/or semiconductive particles; and (b) a layer of neat dielectric polymer or glass in contact with one surface of said layer of variable voltage material; wherein the neat dielectric polymer or glass layer is present in a thickness of less than about 1.6 mils. It has been found that the presence of the thin layer of neat dielectric polymer or glass on the surface of the binder/particle type of variable voltage protection material provides a component having desirable voltage clamping properties, as well as other desirable properties.
In another aspect, this invention provides a layered variable voltage protection component comprising a first layer of variable voltage protection material comprising a binder having dispersed therein at least about 20% by volume of conductive or semiconductive particles; a second layer of variable voltage protection material in contact with the first layer comprising a binder having dispersed therein at least 40% by volume of conductive or semiconductive particles; and a third layer of variable voltage protection material in contact with said second layer comprising a binder having dispersed therein at least 20% by volume of conductive or semiconductive particles. It has been found that the multiple layer construction provides an opportunity to vary the conductor particle loading and/or semiconductor particle loading in each layer, such that the outer layers contain lower particle loadings than the inner layer, in order to achieve a wide range of clamping voltages and other desired properties. In an additional aspect of this invention, it has been found that the outer layer in contact with the electrical conductor of the electronic device should have a lower particle loading than the inner layer with a higher particle loading, but in such case the other outer layer in contact with the ground plane can be higher or lower in particle loading. In an additional aspect of this invention, this multi-layer variable voltage protection component can further be provided with a thin layer of the neat dielectric polymer or glass as referred to above on one outside surface or both outside surfaces, in order to provide additional properties and characteristics of the component. In this aspect of the invention, the layer on the side of the electrical conductor can have a higher or lower particle loading than the inner layer provided the neat dielectric polymer or glass layer is positioned between the outer layer and the electrical conductor. In another aspect of this invention this multiple layer component can be provided with a conductive, e.g., metal, layer interposed between the first layer and second layer and/or between the second layer and third layer of variable voltage protection material. In yet another aspect of this invention, these multiple layer components themselves can be stacked, with or without the outer layers of neat dielectric polymer or glass layers, and with or without an intervening layer of neat dielectric polymer or glass between components to achieve desired performance characteristics.
In another aspect, this invention provides a method of making a variable voltage protection material comprising forming a mixture comprising (a) conductive and/or semiconductive particles and (b) insulating particles in (c) a light organic solvent; mixing said mixture to disperse the insulating particles in the conductive/semiconductive particles; evaporating at least a portion, preferably all, of the solvent; and mixing the resultant mixture of conductive/semiconductive particles and insulating particles with a binder to form a variable voltage protection material.